This invention arises in the field of electronic packages that encase and protect semiconductor circuit devices (dies) and provide the electrical connections that join the die circuitry to external components such as those of a printed circuit board. This invention is particularly concerned with air-cavity packages, i.e., those in which the die resides in an air-filled cavity where the performance of the die benefits from the low dielectric constant of air. In particular, this invention addresses the difficulty of sealing the package around the die and the air-filled cavity in a manner that will maintain a gas-impermeable seal during the high temperatures encountered during the fabrication of the package and the conditions encountered by the package in use.
An electronic package consists of a die sealed inside a protective enclosure whose walls are penetrated by leads by which the die circuitry is electrically connected to external circuitry such as that on a printed wiring board. The packages of interest in this invention are those that are known in the electronics industry as xe2x80x9cair-cavity packagesxe2x80x9d since the die resides in a hollow internal air-filled cavity inside the enclosure, the air serving as an electrical insulator due to its low dielectric constant. This insulating ability is particularly useful when the electronic device is a microwave power chip. The air-filled cavity is also useful when the die is one that requires light transmission, such as CCDs and CMOS devices, since the air provides full optical access to the surface of the die.
To achieve consistent and reliable performance with the extremely fine circuit lines and high current densities that are currently used in dies, the package must be sealed against the intrusion of water vapor and other atmospheric gases. At the same time, the package must be capable of dissipating the heat that the die generates during use. Heat dissipation is commonly achieved through the floor of the package, and for this reason a heat conductive material, generally a metal plate, is used as the floor, with a high-temperature heat-conductive solder, often a eutectic solder, joining the die to the floor. Packages are generally formed by first bonding sidewalls to the metallic plate to form the body of the package, the sidewalls having electric leads passing through them. Once the body is formed, the die is placed inside the body and secured to the floor with the solder. Wire bonding is then done to join the die circuitry to the leads, and the package is finally completed by securing the lid to the body with an appropriate adhesive to close off the top.
The high soldering temperature needed to secure the die to the floor of the package requires that the body of the package be constructed of a material that can withstand the high temperature without cracking, melting, flowing, decomposing or otherwise undergoing transformations that might compromise the seals throughout the package. Packages intended for high-wattage use impose an additional strain on the package walls and lid because of the high temperatures that they generate during use. For these reasons, package sidewalls and lids of the prior art are made of ceramic material. Ceramics are costly, however, and with mass production of the packages, the ceramic is a major component of the manufacturing cost of the package. Cost could be reduced considerably if the ceramic were be replaced by plastic materials, but plastics do not readily withstand the high soldering temperatures and will either melt or decompose when the die is soldered to the base. As a result, the manufacture of electronic packages with plastic sidewalls has a high failure rate.
A similar problem arises in packages that are fabricated as two-piece enclosures, in which the base and side walls are initially formed as a single molded piece of ceramic or plastic with a metallic heat spreader molded or otherwise inserted as the floor, the second piece being the lid. If ceramic is used as the material of construction for the unitary base and sidewalls, the cost is high, and if plastic is used, the product yield in mass production will be low due to deterioration or distortion of the plastic and the formation of leakage sites in a significant portion of the units.
A separate problem is presented by optical packages, i.e., those containing CCDs or CMOS devices that require transparent lids to allow transmission of light. Since these packages do not generate heat in use, they do not require a rapidly heat-dispersing metal base; metal, plastic, or ceramic bases can be used. In addition, the lack of a need for fast heat dissipation eliminates the need for high-temperature metallic soldering allows. Instead, low-temperature soldering can be done using soldering materials such as epoxy. The response to high temperatures is nevertheless a concern, since before the package can be used, it is subjected to further processing subsequent to the assembly of the package itself. This subsequent processing includes soldering of the leads outside the package to external circuitry as well as qualification tests, all of which may involve the use of high temperatures. During exposure to these temperatures, differences in the coefficients of thermal expansion (CTEs) of the package components render the package vulnerable to breakage. In particular, the glass lid that is used on the typical optical package to permit light transmission has a significantly lower CTE than the base, whether the base is metallic, plastic or ceramic. This difference causes the lid and base to expand to different degrees during thermal cycling. Differential expansion causes the package to bow and places the side walls under stress, raising the risk of compromising the seals that bond the sidewalls either to the base or to the lid, or both. When fissures form, the packages will fail the gross leak tests and moisture sensitivity tests that determine whether they are suitable for use, and the yield of useful product (functioning, long-life packages) drops.
The difficulties enumerated above and others that are encountered in the fabrication of air-cavity electronic packages are addressed in accordance with this invention by utilizing at least three initially separate componentsxe2x80x94a base, a sidewall frame, and a lidxe2x80x94to form the enclosure. For packages that generate a high degree of heat during use and are fabricated with a high temperature solder joining the die to the base, the three component construction of the package enclosure permits the die to be soldered to the base before any of the other components of the enclosure are assembled, i.e., before either the sidewalls are bonded to the base or the lid to the sidewalls. Plastic sidewalls can then be bonded to the base with no risk of exposing the plastic to the high temperatures needed for soldering the die to the base. Also, the high cost of ceramics can be eliminated or reduced either by avoiding ceramics entirely or using ceramics only for the base. For vision packages with transparent lids, the three-component construction of the package enclosure permits the use of plastic sidewalls with a non-plastic base and a non-plastic lid. The base and lid can then be formed of materials whose CTEs are close in value while the sidewall is formed of a material with a CTE that differs substantially from those of both the base and lid. The use of plastics having a relatively high CTE as the sidewalls will not place undue stress on the package seals since even though the sidewalls may thicken (i.e., bulge both inwardly and outwardly) at the high temperatures encountered in assembly processing and testing, the base and lid will expand substantially equally, thereby preventing the package from bowing.
This invention thus offers various advantages, depending on the type of package and the materials used. In general, the invention permits wide latitude in the choice of materials while avoiding or reducing the risk of package failure due to fissures resulting from the high temperatures encountered during assembly and use. These and other advantages, features, and embodiments of the invention will be more apparent from the description that follows.